1. Field of the Invention:
The present invention relates to surface treated steel materials having a manganese coating and MnOOH (Manganic hydroxide) formed electrolytically or chemically on the manganese coating, which steel materials show excellent corrosion resistance, workability and weldability, and to a process and an apparatus producing the same.
2. Description of Prior Arts:
As means for providing steel materials, the metallic coatings have been most widely used, and zinc-coated steel materials, in particular, have been and are used in tremendous amounts for manufacturing materials for buildings, automobiles, electric appliances and also used in the forms of wires and sections.
However, as zinc-coated steel materials have been increasingly used in various applications as mentioned above and under severe service conditions, a conventional single zinc coating or single metal coating has not always been able to satisfy requirements and recent trends are that a composite or alloy coating is applied to steel materials so as to improve the properties.
This is due to discoveries and knowledges obtained through long-year experiences that the corrosion protecting effect of zinc (or zinc alloy) based on its nature that it is electrochemically baser than iron, namely due to its sacrificial anodic action, cannot be maintained if the corrosive media is very severe and the dissolution of zinc is so rapid.
For example, referring to a painted galvanized iron, which has been widely used for building materials, a zinc-coated or alloyed zinc-coated steel plate is used.
However, the environments to which the zinc-coated or alloyed zinc-coated steel sheet is exposed usually contain corrosive media, such as water, oxygen and salts, so that the coated zinc dissolves in a very short period of service, thus developing red rust due to the corrosion of the base steel sheet, and further promoting the corrosion of the base steel sheet itself. Therefore, the zinc-coated steel sheet is seldom used without a further surface treatment.
Hereinbelow, mention will be made to steel plates for automobiles, for example. In U.S.A., Canada and European countries, salt is sprayed on highway roads in winter seasons for prevention of freezing of the roads, and the amount of salt to be sprayed has been steadily increasing each year. For this reason, corrosion of the automobile bodies has been an important problem, and the Canadian Department of Consumer and Corporate Affairs has proposed a general guidline in connection with corrosion of the automobile bodies as shown in Table 1 and calls for assistance from the automobile industry.
TABLE 1 ______________________________________ Guideline for Corrosion Protection Proposed by Department of Consumer & Corporate Affairs, Canada 1978 1979 1980 1981 ______________________________________ year year year year No Rust 1 1 1.5 1.5 No Pitting 3 3.5 4 5 No damage on Structural Parts 6 6 6 6 ______________________________________
Meanwhile, the automobile industry has been practising the following corrosion protection measures:
(1) Improvements of pretreatments, such as degreasing and chemical conversion treatments, as well as substitution of the anion type electrodeposition coating;
(2) Improvement of corrosion protecting paints, particularly improvement of resistance to chipping;
(3) Employment of zinc-coated steel materials and zinc-rich paint precoated steel materials.
The measures of the above (1) are useless for portions such as door inner or pointed portions which are accessible to the pretreatments or electrodeposition coating, although effective for the outer skins. Also the measures of the above (3) have defects that when the amount of zinc coating is increased, for example, for improving the corrosion resistance, the weldability and the workability are damaged, while in the case of the precoating, the weldability and the corrosion resistance at worked portions are satisfactory. Therefore, up to now, no satisfactory steel materials are available which can well guarantee the Governmental guidelines shown in Table 1, particularly guarantee of "no pitting" and "no damage" for 5 to 6 years as aimed at in 1981.
Therefore, strong demands have been made for developments of new surface treated steel materials which show far better corrosion resistance than the conventional surface treated steel sheets and at the same time provide workability, weldability and paintability similar to those of ordinary cold rolled steel sheets, all together in a well balanced condition. Therefore, it is an urgent task for the steel industry to satisfy the above demands from the points of safety assurance and material savings.
The corrosive environments to which the automobiles are exposed usually contain corrosive substances, such as water, oxygen and salts, and automobiles are exposed over a long period of time to water and salt confined within their recesses. Therefore, when zinc-coated steel sheets are used in such environments, the coated zinc dissolves in a very short period of time and red rust is caused by the corrosion of the base steel sheet and in more severer cases pitting and damages of structural parts are caused. Thus in the corrosion of automobiles, there is a close relation among the temperature, humidity (time for which the automobile is kept in a wetted condition) and the salt content as has been confirmed by the present inventors. The test results are shown in Table 2 from which it is understood that the salt spray test (JIS-Z-2371) widely used in the steel industry provides the most severe corrosive condition, while the atmospheric exposure test provides the least corrosive condition, and thus the humidity is the most important factor.
TABLE 2 __________________________________________________________________________ Comparison of Corrosion Rates (g/m.sup.2 /year) in Various Environments 5% CaCl.sub.2 + 3% NaCl 5% NaCl + Dry-Wet Atmospheric + Air 0.05% Na.sub.2 SO.sub.4 Repeti- Salt Exposure Exposer + Air Exposure tion Spray Test Test Test Test Test __________________________________________________________________________ Semi-Rural Once a Once a Immersion 3% NaCl District day (15 min) day 15 (min) into 3% 35.degree. C. 3% NaCl spraying NaCl for 100% R.H spraying aqueous 5 min. followed solution drying at by atmos- of above 50.degree. C. for pheric stated 25 min. exposure salts, followed by atmospheric exposure Ordinary 280 1,440 10,500 8,000 7,800- Steel 11,800 Zn 15 60 3,000 180 6,000- 8,640 __________________________________________________________________________
In the salt spray test, zinc dissolves at a corrosion rate of about 1 g/m.sup.2 /hr and if the corrosion resistance is relied solely on the anodic self-sacrificial corrosion protection of zinc, the zinc coating must be made in an amount as large as several hundred grams to one kilogram per square meter, and steel sheets with such a large amount of zinc coating canot be welded, ad the Fe-Zn alloy layer formed between the base steel and the zinc coating is very susceptible to cracking when subjected to workings, such as press forming. This cracking damages the corrosion resistance of such worked portions. Further, from the necessity of energy saving, efforts and trials have been made in reducing the weight of automobiles for the purpose of improving the fuel consumption ratio, and thus it is not desirable to increase the amount of zinc coating indefinitely.
What is more critical matter for the zinc-coated steel sheet is the problem of "contact corrosion" which is caused when the zinc-coated steel sheet is used in combination with an ordinary cold rolled steel sheet as often used in the automobiles. In the automobile industry, the zinc-coated steel sheet is used in combination with a non-coated cold rolled steel sheet into a white body, which is subjected to degreasing, washing, phosphate treatment, electrodeposition paint coating, intermediate coating and upper coating. In this way, when different metals, e.g. zinc and iron are brought into contact with each other in a wetted condition, a galvanic cell is formed between them and promotes dissolution of zinc and as the dissolution is promoted, swelling of the upper paint coating is caused, resulting in damgaes of the paint coating. Thus as shown in FIG. 1, (one sheet of 70.times.100 m/m (A) and another sheet of 70.times.90 m/m (B) were spot welded on two spots, uniformly paint coated and scratched), test pieces by combining a cold rolled steel sheet with a zinc-coated steel sheet by spot welding, and subjecting this combined sheet to a standard phosphate treatment, anionic electrodeposition coating and upper coating, and the test pieces were scratched by a knife cutting the paint coating to the base steel, subjected to 20-day salt spray test (JIS-Z-2371) and the adhesion of the paint coating near the scratched portions was determined by the tape stripping test. The results are shown in FIG. 2. It has been revealed that the adhesion of the paint coating, which is satisfactory good when a cold rolled steel sheet is combined with a cold rolled steel sheet, is definitely lowered near the welded portion between the zinc-coated steel sheet and the cold rolled steel sheet, and this lowered adhesion results in easy peeling-off of the paint coating.
Also zinc-coated steel products are usually subjected to a chemical conversion treatment, such as chromating and phosphating, fitted to the zinc coating, and further subjected to an organic coating compatible to the chemical conversion treatment for the purpose of improving the corrosion resistance and the ornamental value. However, even when the steel products are surface coated by zinc coating, chemical cnversion treatment and organic coating, the zinc coating is first attacked by a corrosive substance, such as water, oxygen and salt which penetrate through the organic coating, and the organic coating itself is damaged by the corrosion product.
As mentioned above, in the case when a zinc-coated steel material having an organic coating on the zinc coating, the corrosion resistance of the zinc coating itself is very important, just as when the zinc-coated steel material is used without an organic coating thereon, and for this reason the recent technical tendency is directed toward inhibition of the sacrificial anodic action of the coated zinc and commercial trials have been made to artificially make the galvanic electrode potential of the zinc coating approach to that of iron by alloying the zinc coating with iron, aluminum, nickel, molybdenum, cobalt, etc. resulting in developments of Zn-Fe alloy coated, Zn-Al alloy coated, Zn-Ni alloy coated, Zn-Mo-Co alloy coated steel products, which are now in the market.
These alloyed zinc coatings are said to have a corrosion resistance two or several times better than that of the conventional zinc coating, but the Zn-Fe alloy coating has difficulty in working, the Zn-Al alloy coating has difficulties in workability, weldability and paintability, and the zinc-nickel alloy coating is hard to obtain in a uniform structure and has a disadvantage that a continuous performance of spot welding is hardly achieved due to its low electric resistance as low as the zinc coating, thus failing to provide a coated material with satisfactorily balanced properties. Although the Zn-Mo-Co alloy coating seems to provide the desired balanced property, it is very difficult to form the alloy coating of uniform composition, because each of the component metals shows a different electrodeposition speed depending on the electroplating conditions.
Therefore, in recent years strong demands have been made in various fields for the balanced property, namely for a comercial development of a surface coated steel material having excellent workability and weldability as well as satisfactory paintability and adaptability to chemical conversion treatments, but up to now, there is no surface coated steel material which can meet with the above requirements.
For improving the corrosion resistance of a steel material by coating the steel material with other metals and utilizing the corrosion resistance of the coated metals, there are two groups of coating methods, as classified electrochemically; the first group in which a metal nobler than iron is coated, for example chromium plating; the second group in which a metal baser than iron is coated, for example, zinc plating. For the first group of methods, many studies have been made and many arts have been established. However, when the metal coating itself has pinholes, or when the thickness of a coating increases, the coating is susceptible to cracking, as seen in the chromium coating. In either case, the metal coating has a defective portion, so that the steel substrate is first attacked because iron is electrochemically baser than the coated metal, just contrary as in the zinc coating, so that pitting corrosion is apt to occur, thus deteriorating the reliability of the coated steel material.
In view of the above facts, it may be concluded that a metal, such as zinc, which shows the sacrificial anodic action is more advantageous for protecting steel materials from corrosion. The present inventors made systematic studies in consideration of the above technical points of view, and have found that among various coated steel materials, a manganese coated steel material having an MnOOH (manganic hydroxide) formed thereon shows the best corrosion resistance. As clearly understood from the galvaic series of metals in an aqueous solution, as manganese is electrochemically baser than zinc, it has been undoubtedly expected that manganese has an inferior corrosion resistance as compared with zinc.
Regarding the electrodeposition of manganese, many various studies have been made including "Electrolytic Manganese and Its Alloys" by R. S. Dean, published by the Ronald Press Co., 1952; "Modern Electroplating" by Allen G. Gray, published by John Willey & Sons Inc., 1953;"Electrodeposited Metals Chap.II, Manganese" by W. H. Safranek, published by Ammerican Elsevier Pub. Co., 1974, and "Electrodeposition of Alloys", Vol. 2 "Electrodeposition of Manganese Alloys" by A. Brenner, published by Academic Press, 1963.
According to R. S. Dean, the electrodeposition of manganese and its alloys act self-sacrificially anodically just as zinc and cadmium in the aspect of rust prevention, and a steel sheet having 12.5.mu. thick manganese coating can well resist to the atmospheric exposure for 2 years, and R. S. Dean reported by citing "Sheet Metal Industry", 29, p.1007(1952) that a satisfactory protective effect can be obtained by a thick manganese coating and that the electrolytic manganese becomes black when exposed to air, but this can be prevented by an immersion treatment in a chromate solution.
Further, according to N. G. Gofman, as reported in "Electrokhim Margantsa" 4, pp.125-141(1969), the electrodeposited manganese corrodes in the sea water at a rate by 20 times faster than zinc, but the corrosion rate of manganese can be decreased when a chromate film is provided on the manganese.
What is more interesting is reported by A. Brenner. He pointed out the following three defects of the manganese or its alloy coatings, although he mentioned a protective film for steels or low alloyed steels as one of the expected applications of the manganese or manganese alloy coatings.
(1) Brittleness PA1 (2) Chemical reactivity (a short service life in an aqueous solution or outdoors) PA1 (3) Dark color of corrosion products (unsuitable for ornamental purposes, yet suitable for a protective coating).
Regarding the brittleness, manganese electrodeposited from an ordinary plating bath, has a crystal structure of .gamma. or .alpha., and the .gamma. structure which is softer transforms into the .alpha. structure when left in air for several days to several weeks. Therefore, in practice, considerations must be given to the .alpha.-manganese. In this case, the hardness and brittleness are said to be similar to those of chromium, i.e. 430 to 1120 kg/mm.sup.2 expressed in microhardness according to W. H. Safranek.
Regarding the chemical reactivity, A. Brenner reported that the manganese or its alloys can be stabilized by a passivation treatment in a chromate solution, and the thus stabilized manganese or its alloys can stand satisfactorily stable for a long period of time in the indoor atmosphere, but he pointed out that for outdoor applications an eutectoid with a metal nobler than manganese should be used.
Therefore, judging from the fact that a zinc coated steel sheet with zinc coating of 500 g/m.sup.2 by hot dipping can protect the steel sheet against corrosion for 30 to 40 years, a zinc coating of 90 g/m.sup.2 by hot dipping which corresponds to a manganese coating of 12.5.mu. can be predicted to resist the atmospheric corrosion at least for 5 to 6 years, therefore a manganese coating which can resist to the atmospheric corrosion for only 2 years cannot be said to have a better corrosion resistance than a conventional surface treated steel sheet.
Up to now no trial or study has ever been made to improve the corrosion resistance of a steel material by manganese coating thereon, except for the invention made by the present inventors as disclosed in Japanese Laid-Open Patent Specifications Sho 50-136243 and Sho 51-75975.
The present invention is clearly distinctive over these prior arts in the following points.
The Japanese Laid-Open Patent Specification Sho 50-136243 discloses a surface treated steel substrate for organic coatings, which is obtained by electro-plating 0.2.mu. to 7.mu. manganese coating on the steel material, and by subjecting the manganese coated steel material to a chromate treatment or a cathodic electro-chemical treatment in a bath of aluminum biphosphate or magnesium biphosphate or both. The technical object of this prior art is to facilitate the conversion treatments by coating manganese because it is difficult to apply in substitution for zinc coating conversion treatments such as the chromate treatment and aluminum biphosphate and magnesium biphosphate treatments directly to the steel material, and also it has an object to improve the paintability and further the corrosion resistance.
The Japanese Laid-Open Patent Specification Sho 51-75975 discloses a corrosion resistant coated steel sheet for automobile, which comprising a steel substrate containing 0.2 to 10% chromium and at least one layer of coating of zinc, cadmium, manganese or their alloys in a total thickness of 0.02.mu. to 2.0.mu.. This prior art is based on the fact that when the chromium content exceeds 0.5%, the crystal formation on the surface becomes increasingly scattered during the phosphate treatment, for example, and when 3% or more of chromium is contained, completely no phosphate crystal is formed, so that an excellent corrosion resistance of a steel substrate can be obtained, and that it is effective to apply only on the steel surface a single layer or multiple layers of coating of zinc, cadmium, manganese or their alloys which are very reactive to the conversion treatments.
As explained above, the prior arts which were also made by the present inventors utilized the nature of manganese that it has a stronger chemical reactivity than zinc for improvement of applicability of a steel material to chemical conversion treatments, and provide a steel substrate for paint coating. Therefore, these prior arts are completely different from the present invention, in which the MnOOH (manganic hydroxide) is intentionally formed on the manganese coating electrolytically or chemically.
Thus the passivation obtained by the conventional chromate immersion is a kind of chemical conversion, just as the chromate treatment usually done on a zinc-coated steel sheet, which is intended to form a chromate film thereby improving the corrosion resistance. Therefore, a large amount of Cr.sub.6+ or Cr.sup.3+ naturally remains in the film. Contrary to this, the electrolytic or chemical treatment in chromic acid used in the present invention is not intended to form a film of Cr.sup.6+ or Cr.sup.3+, but is intended to intentionally promote conversion of the hydrated manganese oxide into the MnOOH(manganic hydroxide) as clearly shown from Table 3. Thus, no Cr ion can be detected in the film of oxyhydrated manganese compound even by the atomic absorption analysis.
The reason why the manganese coating in the prior arts exhibits excellent corrosion resistance is that the thin layer of the oxygen-containing manganese compound formed on the metallic manganese coating is hardly dissolved in water, and serves as a kind of passivated film and contributes to corrosion resistance as contrary to a pure manganese metal which is very reactive.
Thus when metallic manganese is electrochemically deposited using a usual sulfate bath, the metal manganese reacts with oxygen in the air, and manganese hydroxide formed in a thin film during the electro-plating is oxidized by the air and the oxygen-containing manganese compound is formed according to the following formulae (1) and (2). EQU 2Mn(OH).sub.2 +O.sub.2 .revreaction.2H.sub.2 MnO.sub.3 ( 1) EQU H.sub.2 MnO.sub.3 +Mn(OH).sub.2 .revreaction.Mn.multidot.MnO.sub.3 +2H.sub.2 O (2)
This oxygen-containing manganese compound hardly dissolves in a neutral salt solution or in water and provides a very stable corrosion resistant film, completely different from the metallic manganese.
An oxygen-containing metal compound, such as the oxygen-containing manganese compound, is known to contribute to corrosion resistance just as a stainless steel exhibits excellent corrosion resistance due to its passivated surface film of a hydrated oxide containing 20 to 30% water, and a thinly chromium coated tin-free steel exhibits excellent corrosion resistance and excellent paintability due to its oxyhydrated chromium compound film containing about 20% water. It is also known that the rust of steel exposed to the air for a long period of time contains non-crystalline oxyhydrated iron compound, FeOOH, and that the rust layer of an atmospheric corrosion resistant steel which exhibits excellent resistance to atmospheric corrosion contains much of such oxyhydrated iron compound.